1. Technical Field
This disclosure relates to an image forming apparatus including a recording head for jetting liquid droplets, a method of correcting shifts in landing positions of liquid droplets jetted from the recording head, and a landing position shift correction sheet member used for correcting shifts in the landing positions.
2. Description of the Related Art
There are image forming apparatuses such as printers, facsimile machines, copiers and multifunction peripherals having the aforementioned functions for performing image formation (recording, printing, etc., are also used synonymously) by the following method. That is, the image forming apparatus uses a liquid jetting device including a recording head with a liquid jetting head (liquid droplet jetting head) for jetting liquid droplets of a recording liquid (liquid). While a medium (hereinafter also referred to as “sheet”, although the material is not limited, and a recording target medium, a recording medium, a transfer material, a recording sheet, etc., are also used synonymously) is being conveyed, the liquid jetting device jets the recording liquid (liquid) so that the recording liquid adheres onto the sheet.
An apparatus that forms images by jetting liquid onto a medium such as paper, strings, fiber, fabrics, leather, metal, plastic, glass, wood, ceramics, etc., is referred to as an image forming apparatus. Furthermore, “image formation” not only means to form images that have meaning such as characters or figures on a medium, but also means to form images without meaning such as patterns onto a medium. Therefore, textile printing equipment and devices for forming metal wiring are also included. Moreover, the liquid is not particularly limited as long as it is capable of forming images. Furthermore, a device for jetting liquid from a liquid jetting head is referred to as a liquid jetting device, which is not limited to that for forming images.
In such an image forming apparatus employing the liquid jetting method, when printing is performed bidirectionally in forward and backward directions by moving back and forth a carriage provided with a recording head for jetting liquid droplets, the following problem may arise. That is, if the image to be printed includes parallel horizontal lines, the position of a horizontal line printed in a forward direction may be shifted from that of a backward direction (i.e., the lines may not be parallel to each other).
For this reason, when using a typical inkjet recording apparatus, a test chart for adjusting positional shifts of horizontal lines is output manually. The user selects and inputs the optimum value. The timing for jetting ink is adjusted based on the input value. However, different individuals may view the test chart in different ways. Furthermore, if the user is not familiar with this operation, there may be errors in the input data. Therefore, adjustment failures may be caused in this method.
There are conventional techniques for correcting density inconsistencies in image forming apparatuses employing the liquid jetting method. For example, patent document 1 discloses a technique of printing a test pattern onto a recording medium or a conveying belt, reading color data of the test pattern, and changing the conditions for driving the head based on the read results to correct density inconsistencies.
Patent Document 1: Japanese Examined Patent Publication No. H4-39041
Furthermore, patent document 2 discloses a technique for detecting nozzle failures of the liquid jetting head. Specifically, a test pattern, which includes colored dots, which are a combination of cyan ink, magenta ink, and yellow ink, is formed in a predetermined region on a member for holding and conveying a printing medium. This combination of dots is read by an RGB sensor. Based on the read results, nozzles having jetting failures are detected.
Patent Document 2: Japanese Patent No. 3838251
Patent Document 3 discloses a technique of making corrections as follows. A test pattern is recorded onto a part of a conveying belt. The test pattern includes any one of an idling nozzle detection pattern for detecting an idling nozzle, a color shift detection pattern for detecting color shifts of ink, and a head position adjustment pattern for adjusting the position of the recording head, or a combination of these patterns. This test pattern is read by an image pick-up unit such as a CCD, and the correction is made based on this result.
Patent Document 3: Japanese Laid-Open Patent Application No. 2005-342899
Patent Document 4 discloses a technique of detecting the density of toner images with an image forming apparatus employing the electrophotographic method of using toner. Specifically, a toner image is formed on a photoconductive drum. The image forming apparatus includes light emitting elements and light receiving elements for detecting the density of a toner image. The light receiving elements include one for receiving specular reflection light and one for receiving scattered light. These elements individually detect densities of toner images having different characteristics.
Patent Document 4: Japanese Laid-Open Patent Application No. H5-249787
Patent document 5 discloses a technique for detecting the amount of adhering toner. Specifically, a sensor simultaneously detects specular reflection light and diffuse reflection light, which are reflected from a toner image that has been formed. The detection results output from this sensor are used for detecting the amount of adhering toner.
Patent Document 5: Japanese Laid-Open Patent Application No. 2006-178396
However, as described in patent documents 1 through 3, when a test pattern is formed on a conveying belt and the color of the test pattern is detected or the test pattern is read by an image pick-up unit, the following problem may arise. That is, depending on the combination of the color of the conveying belt and the color of the ink, the difference of colors between the conveying belt and test pattern may be small. Thus, it may be difficult to accurately read the test pattern. In such a case, in order to accurately detect the color, it may be necessary to use a light source that changes its wavelength for each color, which increases the cost of the detecting unit. For example, the conveying belt may be an electrostatic belt formed with an insulating layer on its front and a mid-resistance layer on its back, with carbon incorporated to make the mid-resistance layer conductive. The appearance of such an electrostatic belt is black. Therefore, in the process of detecting a test pattern, the electrostatic belt may not be distinguished from black ink, merely with the use of reflections from colors or with an image obtained by an image pick-up unit. For this reason, the test pattern may not be detected with high precision.
More specifically, in the device for correcting density inconsistencies disclosed in patent document 1, the colors are read with a sensor. Therefore, if the color of the jetted ink droplets were similar to the color of the holding/conveying member, the detection precision would decrease. Thus, each color needs to be detected through a filter. This increases the number and types of sensors and filters, which leads to increased costs. Furthermore, the device for detecting nozzle failures disclosed in patent document 2 uses an RGB sensor. Therefore, if the color of the jetted ink droplets were similar to the color of the holding/conveying member, the detection precision would decrease. In order to increase the detection precision, it will be necessary to limit the combinations of the ink and the holding/conveying member. Furthermore, if laser light were to be used, extremely small points would be scanned. For this reason, the detection operation would be affected even by small foreign matter particles or scratches on the conveying member, which decreases the detection precision. If an RGB sensor were to be used, it would be necessary to provide at least one unit for reading each of the colors, which leads to increased cost. Moreover, the device disclosed in patent document 3, which uses the image pick-up unit, has the same problem as that of patent document 2. That is, if the color of the jetted ink droplets and the color of the holding/conveying member were similar, the detection precision would decrease. Furthermore, the image is recognized as a two-dimensional image, which requires a processing system with higher performance than the case of recognizing a one-dimensional image, which leads to increased costs.
Accordingly, the methods disclosed in patent documents 4 and 5 for detecting the adhering toner amount in the electrophotographic method may be applied. Even when toner particles contact each other, the shape of each particle is maintained. For this reason, it is possible to read the toner density even at portions where the toner is so concentrated that it is piled up along a rectangular line. However, in the case of liquid droplets, the droplets cohere to each other. Therefore, if these methods (of patent documents 4 and 5) were to be directly applied to an image forming apparatus employing the liquid jetting method, it would be possible to perform the detection, but only to the extent of detecting noise. Thus, the test pattern may not be detected with high precision.
Furthermore, in a case where the test pattern is formed on plain paper, which is a recording target medium into which ink can permeate, and the test pattern is read by an optical sensor, the following problem arises. That is, the ink will permeate into the paper and form smears, such that the pattern becomes blurred. As a result, the landing positions may not be accurately detected.